lib: Add a new platform feature to bringup secondary harts

A platform may have a specific method to bring-up secondary harts for the
first time but may rely on generic WFI based approach for hart hotplug in
absence of a platform specific hart hotplug method.

Define a platform feature flag for such platforms. The platform needs to
implement platform specific bring-up method in hart_start and not define
hart_stop method in this case. They must only define
SBI_PLATFORM_HAS_HART_SECONDARY_BOOT.

SBI_PLATFORM_HAS_HART_HOTPLUG should only be defined when the platform
intend to support both hart_start and hart_stop and do not intend to rely
on generic WFI based approach.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>